1. Field of the Invention
The invention relates to measuring inter-frequencies in a mobile telephone system employing frequency division duplex (FFD) and especially to defining measurement gaps generated for making said measurements in a code division multiple access (CDMA) system.
2. Brief Description of Related Developments
Third-generation mobile telephone systems called UMTS (Universal Mobile Telephone System) and IMT-2000 (International Mobile Telephone System), for instance, will use wideband code division multiple access technology, i.e. WCDMA technology, on the radio path. In a WCDMA system, all mobile stations in a cell use the same frequency between each other on the transmission link from the mobile station to the base station and correspondingly, the same frequency between each other on the transmission link from the base station to the mobile station. A WCDMA system can in mobile telephone systems be implemented either as frequency division duplex (FDD) or time division duplex (TDD).
In an FDD-type WCDMA system, the uplink direction (from the mobile station to the base station) and the downlink direction (from the base station to the mobile station) transmissions are independent of each other. Thus, the base stations need not be synchronized with respect to each other, either. It is, however, typical of CDMA systems that a downlink transmission is performed simultaneously from several base stations to one mobile station, which transmission the receiver of the mobile station is arranged to receive. This arrangement is called a soft handover, and to control it, the mobile station must perform various parameter measurements for both uplink and downlink connections. Corresponding measurements are also used in updating the location of a mobile station and in handovers between WCDMA and GSM systems.
The receiver of a mobile station is typically arranged to receive only one frequency at a time, which means that one set of receiving means is enough for the mobile station and there is no need to design antenna diversity to them, which is advantageous both in view of cost and making the structure of the mobile station simple. The mobile station can also be designed to comprise several receiving means (dual receiver), which usually include antenna diversity. This type of mobile station is, however, more expensive and complex to implement.
Thus, the parameter measurements described above can be performed in a typical one-receiver mobile station only when there is no transmission. This also applies to dual-receiver mobile stations when one set of transmission/reception means transmits on almost the same frequency as a second set of transmission/reception means performs measurements. In an FDD-type WCDMA system, the transmission is interrupted for a while by generating in a frame a gap during which transmission is interrupted. This is done by using what is known as compressed mode or slotted mode in which information normally transmitted in a 10-ms frame is transmitted in a shorter time. Since the same information is transmitted in a shorter time, a gap remains in the frame, during which measurements of the parameters described above can then be performed. Depending on the measurement situation and the transmitter properties, compressed mode is only used in uplink or downlink transmissions, or a combined uplink/downlink compressed mode can also be used.
In compressed mode, a gap can be generated into the transmission in at least three ways: by puncturing the data being transmitted, by halving the spreading factor, or by buffering the data being transmitted onto higher protocol layers for a while. One of the above compressed mode methods is signalled to the mobile station for use. Up to a third of the transmitted bits can be removed with the puncturing methods used in a WCDMA system, whereby a gap of up to five time-slots can in compressed mode be generated into a frame comprising 15 time-slots. A gap of this length is, however, often impossible to generate, because puncturing is also used to adapt data rates in an ordinary transmission, which means that this takes up a part of the puncturing capacity and the compressed mode gap becomes shorter than five time-slots. By halving the spreading factor, it is possible to double the data rate, and a gap of up to seven time-slots can be generated in a frame of 15 time-slots. In such a case, transmission power must be increased to keep the signal-to-interference ratio of the received signal substantially constant. Buffering data onto higher protocol layers is only possible with non-real-time connections, such as with packet data transmissions having low quality of service (QoS).
A problem with the above arrangement is that in most measurements, such as in handover measurements between UMTS and GSM, for instance, a longer measurement gap would be more advantageous than the gap of at most seven time-slots mentioned above. A longer gap can be generated by placing two gaps after each other so that the first gap is at the end of the first time-slot frame and the second gap is at the beginning of the next time-slot frame. When using the puncturing method, it is this way possible to generate a gap of at most 10 time-slots, but maximum puncturing is not always possible in compressed mode, due to a possible data rate adaptation. By halving the spreading factor, it is possible to generate a gap of up to 14 time-slots, but then the transmission rate must be increased during two frames, which causes interference to the transmissions of other mobile stations in the same cell and consequently, they, too, need to increase their transmission power to compensate for the interference. Buffering data onto higher layers cannot be used with real-time connections.